A tracked vehicle's weight is transferred to the bottom length of track by a number of road wheels, or sets of bogie wheels. Road wheels are frequently mounted on some form of suspension to cushion the ride over rough ground. Suspension design in military vehicles is a major area of development, and the very early designs were often completely un-sprung. Later-developed road wheel suspension offered only a few inches of travel using springs, whereas modern hydro-pneumatic systems can provide several feet of travel and typically incorporate shock absorbers. Torsion-bar suspension is probably the most common type of military vehicle suspension. Construction vehicles have smaller road wheels that are designed primarily to prevent track derailment, and they are normally contained within a single bogie that integrates the idler wheel and sometimes the drive sprocket.
Track drive suspensions are inherently faced with performance issues which include limited travel/effective suspension, track derailment, and restricted/limited utilization of attachments. These limitations have a direct effect on machine directional/steering control, ride quality, flexibility/functionality with attachments, stability and travel speed.
Transfer of power to the tracks is accomplished by drive wheels (friction), or drive sprockets, that are powered by transmissions or motors that engage holes or lugs in the track links that drive the track. In military vehicles, the drive wheel is typically mounted well above the contact area on the ground, allowing it to be fixed in position. In agricultural and construction tracked vehicles, the drive wheel is normally incorporated as part of the bogie. Placing suspension on the drive sprocket is possible, but is mechanically more complicated. A non-powered wheel, an idler, is placed at the opposite end of the track, primarily to tension the track-loose track could be easily thrown (slipped) off the wheels. To prevent throwing, the inner surfaces of the tracks usually have vertical guide lugs engaging gaps between the bogie and idler/sprocket wheels. In military vehicles with a rear sprocket, the idler wheel is placed higher than the road wheels to allow it to climb over obstacles. Some track arrangements use return rollers to keep the top of the track running straight between the drive sprocket and idler. Others, called slack track, allow the track to droop and run along the tops of large bogie (sometimes called road) wheels. This was a feature of the Christie suspension, leading to occasional misidentification of other slack track-equipped vehicles. Many WW II German military vehicles, including all half-track and all later tank designs (after the Panzer IV), had slack-track systems, usually driven by front-located drive sprockets, running along the tops of the often overlapping, and sometimes interleaved, large diameter doubled road wheels (on the Tiger I and Panther, in their suspension systems). The choice of overlapping/interleaved road wheels allowed the use of slightly more torsion bar suspension members, allowing any German tracked military vehicle with such a setup to have a noticeably smoother ride over challenging terrain, but at the expense of mud and ice collecting between the overlapping areas of the road wheels, and freezing solid in cold weather conditions, often immobilizing the vehicle so equipped.
It takes considerable power to steer a tracked vehicle. As the vehicle turns, the leading and trailing ends of the footprint, or contact patch, skid sideways, perpendicular to the direction the tracks roll. Hence the name “skid steering” could be applied.
In FIG. 1, the arrows indicate the direction in which the contact patch will move during a right (clockwise) neutral axis (Zero) turn. A neutral axis (Zero) turn is a turn about a center point through the machine or the powered drive axle. The further toward the ends, the more the track will move in a direction other than the direction in which it would normally move for forward propulsion.
FIG. 2 shows the magnitude of the frictional forces that must be overcome in order to make the vehicle turn about its vertical axis. These are simply the horizontal component of the direction that each point of the contact patch will move as the vehicle rotates. The friction at any point is proportional to the distance forward of the vertical axis. From this it follows that the total force required is proportional to the length of the contact patch, the weight of the vehicle, and the inverse of the radius of the turn.
The worst-case scenario for overcoming friction is the pivot turn. A pivot turn is a turn about a center point through the center of a “stationary” traction track. In a pivot turn, in which one track travels in a direction while the other track stays stationary, which results in the vehicle rotating about a center point through the center of a “stationary” traction track.
Further, turns executed while both tracks are traveling generally require less power, as less energy is required to overcome the static friction associated with a travelling track, as opposed to a static track. Also, apart from the pivot turn, when compared to the zero turn, turns of greater radii will require less power, as the energy required to overcome the static friction (or terrain abrasion) is spread out over a longer period of time.
Therefore, a need exists for an improved suspension system for tracked vehicles.